This invention relates generally to load responsive fluid control valves and to fluid power systems incorporating such valves, which systems are supplied by a single fixed or variable displacement pump. Such control valves are equipped with an automatic load responsive control and can be used in a multiple load system in which a plurality of loads is individually controlled under positive load conditions by separate control valves.
In more particular aspects this invention relates to direction and flow control valves capable, in a proportional way, of controlling simultaneously a number of loads under positive load conditions.
In still more particular aspects this invention relates to pilot operated load responsive controls of direction control valves, which permit variation in the level of control differential between pump discharge pressure and the load pressure signal, while this control differential is automatically maintained constant at each controlled level.
In still more particular aspects this invention relates to pilot operated load responsive controls of direction control valves, which permit variation in the controlled pressure differential between valve inlet pressure and the load pressure, in response to an external control signal.
Closed center load responsive fluid control valves are very desirable for a number of reasons. They permit load control with reduced power loss and therefore, increase system efficiency and when controlling one load at a time provide the proportional feature of flow control, irrespective of the variation in the magnitude of the load. Normally such valves include a load responsive control, which automatically maintains pump discharge pressure at a higher level, by a constant pressure differential, than the pressure required to sustain the load. A variable orifice, introduced between pump and load, varies the flow supplied to the load, each orifice area corresponding to a different flow level, which is maintained constant, irrespective of the variation in magnitude of the load. The application of such a system is however, limited by one basic system disadvantage.
Normally in such a system the load responsive valve control can maintain a constant pressure differential and therefore constant flow characteristics when operating only one load at a time. With two or more loads, simultaneously controlled, only the highest of the loads will retain the flow control characteristics, the speed of actuation of lower loads varying with the change in magnitude of the highest load. Fluid control valve for such a system is shown in U.S. Pat. No. 3,488,953 issued to Haussler.
This drawback can be overcome in part by the provision of a proportional valve, as disclosed in my U.S. Pat. No. 3,470,694, dated Oct. 7, 1969 and also in U.S. Pat. No. 3,455,210 issued to Allen on July 15, 1969. However, while those valves are effective in proportionally controlling multiple positive loads at a time, they provide a constant pressure differential and therefore a constant throttling loss across each valve, thus reducing system efficiency. Also those valves use an unamplified load pressure signal in operation of their controllers requiring a control signal at a comparatively large energy level.